Oblivion Gates in the game start spawning after you progress the main quest to the point where you deliver Martin to Jauffre at Weynon Priory. Specifically, the first gate spawns around the time you rescue Martin during the quest "The Siege of Kvatch." At that point, about 10 Oblivion Gates will spawn at fixed locations across Cyrodiil. Additionally, more gates will gradually spawn the further you progress in the main questline. In total, there can be as many as 60 gates spawned at one time, but they do not all appear simultaneously. There are approximately 100 designated spawn locations where gates may randomly appear. You do not need to close all gates to finish the game, but if you want to stop any new gates from appearing, you need to complete at least around 20 of them and advance to the end of the main questline. The gates spawn as part of the main quest progression and may appear in cities and wilderness areas over time after Martin is brought to the priory. So to summarize:
- Oblivion Gates start spawning after you deliver Martin to Jauffre at Weynon Priory.
- About 10 gates spawn immediately at fixed locations, with more spawning later to a max of around 60 at once.
- Gates spawn at up to 100 possible locations randomly.
- You can stop new gates from spawning by completing about 20 of them and finishing the main questline.
